False alarm inhibit circuit in echo ranging systems

ABSTRACT

Two or more filters with narrow side-by-side passbands will each pass broadband noise voltages but only one filter can pass a useful narrowband signal. An exclusive OR gate will pass a single filter output but will block simultaneous noise outputs thus effectively distinguishing between useful signals and false alarms.

United States Patent [72] Inventor Lewis II. Bnssert El Cajon, Calif.

[211 Appl. No. 845,444

[22] Filed July 28, I969 [45] Patented Sept. 28, I971 The United Statesof America as represented by the Secretary of the Navy [73] Assignee[54] FALSE ALARM INHIBIT CIRCUIT IN ECHO 343/5 DP, 7 RS, 17.1

[56] References Cited UNITED STATES PATENTS 3,320,576 5/1967 Dixon etal. 340/5 Primary ExaminerRichard A Farley Attorneys-Joseph C. Warfield,Jr., George J. Rubens, John W. McLaren and Truman L. Styner ABSTRACT:Two or more filters with narrow side-by-side passbands will each passbroadband noise voltages but only one filter can pass a usefulnarrowband signal. An exclusive OR gate will pass a single filter outputbut will block simultaneous noise outputs thus effectivelydistinguishing between useful signals and false alarms.

MATCHED FILTER BANK MF No.1

7* RECEIVER ins AND OUTPUT INHIBITS OR OUTPUT MF No.2 7 FOR. [f (fond) I2 a n EXCLUSIVE 24 L] i i (m OR o (firm) GATE 1/8 20 M F No. 3 7 FOR ti1 FALSE ALARMINIIIBIT CIRCUIT IN ECHO RANGING SYSTEMS STATEMENT OFGOVERNMENT INTEREST The invention described herein may be manufacturedand used by or for the Government of the United States of America forgovernmental purposes without the payment of any royalties thereon ortherefor.

BACKGROUND OF THE INVENTION In most sonar, radar and communicationsreceivers, a signal voltage threshold is established above which allsignal voltages are classed as legitimate signals and below which allsignal voltages are classed as noise. In a receiver where thesignal-tonoise ratio is unfavorable, and where sensitivity is important,the threshold is usually established as close to the noise level aspossible and still efiectively discriminate between random noise andtrue signals. Any strong random signal appearing above the thresholdwhich is not a true signal must be classified as a false alann. Ingeneral, the threshold level is related to the maximum false alarm rateacceptable in the particular system of interest. 7

The object of this invention is to provide an improved receiver whichwill effectively discriminate between useful signals and false alarmsignals of the same frequency.

SUMMARY OF THE INVENTION The objects of this invention are attached byproviding two or more filters with narrow side-by-side passbands. Eachfilter will pass the basic carrier frequency of the system or closelyadjacent doppler shifted components thereof, but the broadbands of noisevoltages will pass simultaneously through some or all of the filters. Anexclusive OR gate connected to the output of all of the filters, willpass a single filter output but will block simultaneous noise voltagesat the outputs of the several filters.

Other objects and features of this invention will become apparent tothose skilled in the art by referring to the preferred embodimentsdescribed in the following specification and illustrated in theaccompanying drawing in which the block diagram illustrates theessential elements of the preferred embodiment.

At 10 in the drawing is shown a receiver for receiving, amplifying andfiltering sonar, radar, telemetry or other communication signals pickedup at the antenna or transducer 12. Automatic gain control and/or alimiting function is performed in the receiver to substantially levelthe signals.

In the case of active radar or sonar, pulses of locally generatedfrequencies, f,,, are transmitted outward to the field to beinterrogated. Echo signals reflected from distant objects are sensed bythe sensor 12 and are employed to identify the object as well asdetermine the range of the target. The frequency of the received signal,however, might vary slightly from the frequency of the outgoing orinterrogating signal because of relative motion between the targetobject and the receiver 10, or because of reverberations or variationsof the paths taken by the outgoing and incoming signals. Variable skipdistance of radio signals or erratic surface and bottom bounce for sonarsignals will cause a doppler shift, f,,, of the received frequencies.The upward doppler shift and the downward doppler shift will result,respectively, in j',,+ and fl-f at the receiver.

According to an important feature of this invention, the threenarrowband filters l4, l6 and 18 are coupled in parallel to the outputof the receiver to pass, respectively, the center or basic frequency,f,,, and the upper and lower doppler shifts. Where closely adjacentnarrowbands are to be separated it is preferred that matched filters beemployed at 14, 16, and 18. Such filters may be of the type illustrated,for example, in the Erickson U.S. Pat.'No.. 3,249,940 where a delay linewith a series of lumped inductances are connected at spaced points toinverters to produce a series of coded plus ones and minus ones at theoutput of the filter as a received Tplulse progresses from one end 0 thedelay line to the other. e outgoing and received series of plus ones andminus ones can be cross correlated to retrieve signals buried deep innoise.

A received echo signal will normally be expected to pass either filter14 at frequency L, or be shifted upwardly or downwardly and passedthrough filter 16 or 18. ln any case the useful signal would appear onlyat the output of one of the filters.

Noise pulses, however, are usually broadbanded and any noise pulse whichexceeds the threshold of the receiver will contain component frequencieswithin all three bands f j',+f,,, and fl,j},.

The logic circuit shown in the right hand portions of the FIGURE takesadvantage of this bandwidth difference between the noise and the usefulsignal. The exclusive OR gate 20 is connected between the outputterminal 24 and the output of each of the filters so that a signal atthe output of only one of the filters will occur at the output terminal24. If, however, a signal appears simultaneously, or nearlysimultaneously, at two or more outputs and hence at the input of ANDgate 22, the OR gate 20 is disabled to prevent passage of any signal.Hence, the ratio of false alann to useful signals is materially reduced.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. lt is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

l. A false alarm inhibit circuit in an echo ranging system, said circuitcomprising:

a plurality of narrow passband filters connected in parallel to theoutput of an echo receiver, and being responsive, respectively, tofrequencies equal to and slightly removed from the transmittedfrequency, 1),, of said system so that one only of said filters willpass said transmitted frequency but all filters can pass broadbandnoise,

a logical exclusive OR gate circuit coupled to the outputs of saidfilters,

means responsive to all said outputs for disabling said gate when signalvoltages occur simultaneously on all said outputs and for enabling saidgate when a signal voltage occurs on one only of said outputs.

2. In the false alarm inhibit circuit defined in claim 1,

said filters comprising matched filters resonant, respectivey fihf d fi)f1 wherein f, comprises a doppler shift frequency component.

3. In the false alarm inhibit circuit defined in claim I;

said means comprising an AND gate with plural inputs connected andresponsive to the outputs of all of said filters and a single outputconnected to said OR gate to produce an inhibiting pulse which disablessaid OR gate.

1. A false alarm inhibit circuit in an echo ranging system, said circuitcomprising: a plurality of narrow passband filters connected in parallelto the output of an echo receiver, and being responsive, respectively,to frequencies equal to and slightly removed from the transmittedfrequency, fo, of said system so that one only of said filters will passsaid transmitted frequency but all filters can pass broadband noise, alogical exclusive OR gate circuit coupled to the outputs of saidfilters, means responsive to all said outputs for disabling said gatewhen signal voltages occur simultaneously on all said outputs and forenabling said gate when a signal voltage occurs on one only of saidoutputs.
 2. In the false alarm inhibit circuit defined in claim 1, saidfilters comprising matched filters resonant, respectively, to fo, fo+fd,and fo-fd, wherein fd comprises a doppler shift frequency component. 3.In the false alarm inhibit circuit defined in claim 1; said meanscomprising an AND gate with plural inputs connected and responsive tothe outputs of all of said filters and a single output connected to saidOR gate to produce an inhibiting pulse which disables said OR gate.